1. Field of the Invention
The present invention relates generally to electrical wiring devices, and particularly to protective wiring devices.
2. Technical Background
An electrical distribution system provides power to a house, building or some other facility. Electrical wiring from a power utility provides AC power to one or more breaker panels disposed in the structure. The breaker panel distributes AC power to one or more branch electric circuits installed in the structure. The electric circuits may typically include one or more receptacle outlets and may further transmit AC power to one or more electrically powered devices, commonly referred to in the art as load circuits. The receptacle outlets provide power to user-accessible loads that include a power cord and plug, the plug being insertable into the receptacle outlet. However, certain types of faults have been known to occur in electrical wiring systems. Accordingly, each electric circuit typically employs one or more electric circuit protection devices.
There are several types of electric circuit protection devices. For example, such devices include ground fault circuit interrupters (GFCIs), ground-fault equipment protectors (GFEPs), and arc fault circuit interrupters (AFCIs). This list includes representative examples and is not meant to be exhaustive. Some devices include both GFCIs and AFCIs. As their names suggest, arc fault circuit interrupters (AFCIs), ground-fault equipment protectors (GFEPs) and ground fault circuit interrupters (GFCIs) perform different functions.
A ground fault occurs when a current carrying (hot) conductor creates an unintended current path to ground. A differential current is created between the hot/neutral conductors because some of the current flowing in the circuit is diverted into the unintended current path. The unintended current path represents an electrical shock hazard. A ground fault may occur for several reasons. First, the hot conductor may contact ground if the electrical wiring insulation within a load circuit becomes damaged. This scenario represents a shock hazard. For example, if a user comes into contact with a hot conductor while simultaneously contact ground, the user will experience a shock. A ground fault may also occur when the equipment comes in contact with water. A ground fault may also result from damaged insulation within the electrical power distribution system.
A ground fault creates a differential current between the hot conductor and the neutral conductor. Under normal operating conditions, the current flowing in the hot conductor should equal the current in the neutral conductor. Most GFCIs are configured to take advantage of this fact by comparing the current in the hot conductor to the current in the neutral conductor by sensing the differential current between the two conductors. The GFCI is sensitive to differential currents that represent an imminently dangerous condition, such as the presence of a current path other than the intended path of normal operation (i.e., a shock hazard). Accordingly, when the differential current exceeds a predetermined threshold, usually about 6 mA, the GFCI typically responds by interrupting the circuit. Circuit interruption is typically effected by opening a set of contacts disposed between the source of power and the load. The GFCI may also respond by actuating an alarm of some kind.
One problem associated with protective devices relates to the device being miswired in the filed by an installer. Miswiring refers to a situation wherein the installer couples the line terminals to the load and couples the load terminals to the AC power source. Miswiring may result in the protective device not protecting the user from the fault conditions described above. Labels and installation instruction sheets have been used to prevent miswiring. However, instructive material may be ignored by an installer.
Another problem is that protective device, like all electrical devices, have a limited life expectancy. When the device has reached end of life, the user may not be protected from the fault condition. Test buttons have been incorporated into protective devices to provide the user with a means for testing the effectiveness of the device. One drawback to this approach lies in the fact that if the user fails to use the test button, the user will not know if the device is functional.
Transient voltages are known to damage a protective device/cause an end of life condition. As a frame of reference, the power source of the electrical distribution system includes at least one phase voltage that propagates by way of the hot line in a properly wired device. Transient voltages, on the other hand, may likewise propagate in the electrical distribution system. However, the peak amplitudes of the transient voltages are typically greater than the phase voltage amplitude by an order of magnitude. Transient voltages may be generated in a number of ways. For example, transient voltages may be generated by lightning storms. Transient voltages may also be produced when an inductive load coupled to the electrical distribution system is turned off, or by a motor coupled to the electrical distribution system that includes commutators and brushes. Whatever the cause, transient voltages are known to damage a protective device and cause an end of life condition.
What is needed, therefore, is a protective device that addresses the miswiring issue by detecting the wiring state of the device and inhibiting device operation if the device is miswired. What is also needed is a protective device that includes circuitry that more effectively guards against damage from transient voltage surges.